In a switching power supply of the step-down type employing an N-channel transistor for a power device, the power transistor is driven by a bootstrapping or charge-pump voltage boosting technique. FIG. 1 herewith is a basic diagram of a bootstrapped power supply 1.
The above technique comprises charging a capacitor, called the bootstrap capacitor, which is placed between the output of the power device and the supply VDrive to the drive circuit or driver of the device. In FIG. 1, this bootstrap capacitor is referenced Cb. The supply voltage to the drive circuit is approximately the combined values of the supply voltage VPow to the power device T1 and the voltage at the capacitor Cb, i.e.:VDrive=VPow+VCb−VD0.
Thus, the voltage drop Vgs across the gate and source terminals of the power device T1 is always near-constant under varying conditions of operation, i.e., on/off switching of the power device always provides good overdrive.
Assuming the capacitor Cb to be in a charged initial state, when the power device T1 is turned on, the node Vpow is at Vcc, but the power device will stay on, since voltage VDrive is equal to Vcc+Vcb−VD0.
Also, with the power device T1 on, a current will be circulated such that, as the power device is switched off, the ratio dIl/dt makes the diode D1 conductive and VPow≈0V. Accordingly, the capacitor Cb will be charged by the voltage generator connected to it through the link 2 comprised of components VCb, D0, Cb, and D1. Of course, this operation would be feasible only when the coil contains sufficient energy to pull the cathode of the diode D1 below ground.
The fundamental law for inductors, ΔVL=−L·dIL/dt, indicates that, in the above instance, with the output current IOut being small, ΔIL will be low, and Δt finite, so that, when the power device changes over, the voltage variation across the coil will be insufficient to pull the cathode of diode D1 below ground due to parasitic capacitances. Therefore, the capacitor Cb cannot be charged within time Toff, and will keep being discharged due to a continual current draw from the drive circuit.
Thus a condition is ultimately reached of the supply voltage to the drive circuit being unable to drive the power device T1 as expected.
An attempt at overcoming this problem is represented by European Patent Application No. EP 0 822 475, which is herein incorporated by reference.
However, the proposal of that patent application cannot overcome the problem at 100% duty cycle.
FIG. 2A is a plot with respect to time for a number of signals that are present in the power supply described in the above patent application. These signal plots clearly show that the above patent application will only drive the duty cycle of the PWM signal to 100% for one period, this PWM signal being the control signal to the power device T1. Basically, the coil current is raised to a sufficient ΔV for the cathode of diode D1 to be pulled to ground and the capacitor Cb charged.
At the following cycle, when the duty cycle is driven to 0%, the increase in the coil current ΔIL is large enough at low duty cycles to produce a voltage differential ΔVL that can place the diode D1 in forward conduction (proper operation).
FIG. 2B shows that at high duty cycles, the relative increase in the current ΔIL is so small that the voltage differential ΔVL is inadequate to place the diode D1 in the forward mode. Thus, the aforementioned patent application cannot overcome the problem when conditions are as outlined above.
Consequently, there is a need for a power supply that can keep the bootstrap capacitor charged under conditions of a small current IL and a very high duty-cycle value (close to 100%). As discussed above, with prior art circuits the current will be so small that in this case it cannot pull the cathode of the loop diode below ground, and thus will inhibit the charge current to the bootstrap capacitor.